Focal length differences

Started 1 month ago | Discussions thread
A Marcus Forum Member • Posts: 50
Re: Focal length differences

Camera lenses of yesteryear were simple; often made using just one glass lens. Today, lenses are more complex being made using several individual glass lenses, each called an element. A modern lens maker attempts to mitigate naturally occurring lens defects by constructing the camera lens using several glass lens elements. Some are convex (positive power), some are concave (negative power). Some are made using dense glass material, some using lightweight glass. Some of the elements are free standing, some are cemented together. All this and more is used to mitigate the seven major optical defects that plague. These are called aberrations.

When the camera lens is finished, it is a complex array of glass. In a zoom lens, some of these elements move, changing their spacing as you zoom. If the lens were simple, we could find the center of the lens barrel and make our measurements. The lens to image distance is the focal length. The lens to subject distance is called the object distance. As most lenses are complex, we measure the object distance from a point called the front nodal. We measure the focal length from the rear nodal to the image plane, when the camera is focusing or a far distant subject (infinite distance symbol ∞).

The front and rear nodals are a point on the lens axis (line through the center of the barrel). One would think the front nodal is first followed by rear nodal. However in some designs, the position of these nodals is actually flipped. In a true telephoto design, the rear nodal is caused to fall far forward of the center of the barrel. Such a design causes the lens barrel to be far shorter than you might think. This foreshortened barrel makes the telephoto lens less awkward to use.

In some extreme wide-angle lenses, the focal length is too short to allow the lens to be mounted, so that room is allowed for mechanisms like reflex mirrors and light measuring arrays. Thus wide-angle lenses often have the rear nodal fall further to the rear; this provides more space between lens and camera.

So the answers is this: lens maker can shift these two cardinal points as needed for a given task.

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